ABSTRACT The kinetic and structural behavior of a photochromic compound, 3-(2- fluorophenyl)-3-phenyl-3H-naphtho [2, 1-b] pyran (F-Py), was investigated using 1 H and 19 F nuclear magnetic resonance (NMR) spectroscopy. Upon irradiation, the four theoretically predicted photomerocyanines appear along with a fifth form X, whose final structure has not been elucidated. This last form and two of the photomerocyanines are thermally labile, ...